provide end-user hints and show them

Only a few for now.

lib/mu-store.cc

@@ -241,7 +241,8 @@ Store::Store(const std::string& path, Store::Options opts)
 		if (s_version < 500)
 			throw Mu::Error(Error::Code::CannotReinit,
 					"old schema ({}) is too old to re-initialize from",
-					s_version);
+					s_version).add_hint("Invoke 'mu init' without '--reinit'; "
+							    "see mu-init(1) for details");
 		const auto old_root_maildir{root_maildir()};
 
 		MemDb mem_db;
@@ -258,7 +259,8 @@ Store::Store(const std::string& path, Store::Options opts)
 	if (s_version != ExpectedSchemaVersion)
 		throw Mu::Error(Error::Code::SchemaMismatch,
 				"expected schema-version {}, but got {}",
-				ExpectedSchemaVersion, s_version);
+				ExpectedSchemaVersion, s_version).
+			add_hint("Please (re)initialize with 'mu init'; see mu-init(1) for details");
 }
 
 Store::Store(const std::string& path,
@@ -348,7 +350,7 @@ Store::add_message(Message& msg, bool use_transaction, bool is_new)
 	// we shouldn't mix ngrams/non-ngrams messages.
 	if (any_of(msg.options() & Message::Options::SupportNgrams) !=
 	    any_of(message_options() & Message::Options::SupportNgrams))
-	    return Err(Error::Code::InvalidArgument, "incompatible message options");
+		return Err(Error::Code::InvalidArgument, "incompatible message options");
 
 	/* add contacts from this message to cache; this cache
 	 * also determines whether those contacts are _personal_, i.e. match

lib/mu-xapian-db.hh

@@ -77,7 +77,8 @@ xapian_try_result(Func&& func) noexcept -> std::decay_t<decltype(func())>
 try {
 	return func();
 } catch (const Xapian::DatabaseLockError& dlerr) {
-	return Err(Error::Code::StoreLock, "database locked");
+	return Err(Error{Error::Code::StoreLock, "database locked"}.
+		   add_hint("Perhaps mu is already running?"));
 } catch (const Xapian::Error& xerr) {
 	return Err(Error::Code::Xapian, "{}", xerr.get_error_string());
 } catch (const std::runtime_error& re) {

mu/mu.cc

@@ -61,26 +61,11 @@ handle_result(const Result<void>& res, const Mu::Options& opts)
 		mu_printerrln("{}{}{}",
 			      col.fg(Color::BrightBlue), res.error().what(), col.reset());
 
-	mu_printerr("{}", col.fg(Color::Green));
-
 	// perhaps give some useful hint on how to solve it.
-	switch (res.error().code()) {
-	case Error::Code::InvalidArgument:
-		break;
-	case Error::Code::StoreLock:
-		mu_printerrln("Perhaps mu is already running?");
-		break;
-	case Error::Code::SchemaMismatch:
-		mu_printerrln("Please (re)initialize with 'mu init'; see mu-init(1) for details");
-		break;
-	case Error::Code::CannotReinit:
-		mu_printerrln("Invoke 'mu init' without '--reinit'; see mu-init(1) for details");
-		break;
-	default:
-		break; /* nothing to do */
-	}
-
-	mu_printerr("{}", col.reset());
+	if (!res.error().hint().empty())
+		mu_printerrln("{}hint{}: {}{}{}",
+			      col.fg(Color::Blue), col.reset(),
+			      col.fg(Color::Green), res.error().hint(), col.reset());
 
 	return res.error().exit_code();
 }